In subterranean wells, such as oil and gas wells, there are occasions when material, such as cement slurry or other chemicals, need to be introduced into the well bore. One common example is the introduction of cement slurry into a well bore to seal the well bore or the introduction of cement slurry above a bridge plug to seal off a section of the well bore. This is typically accomplished by what is commonly known in the industry as a dump bailer. Dump bailers are introduced or carried into a subterranean well on a conduit, such as wire line, electric line, continuous coiled tubing, threaded work string, or the like, and discharge or “dump” the cement slurry into the well bore.
There are two general types of dump bailers: (1) gravity feed bailers and (2) positive displacement bailers. Gravity feed dump bailers are some of the most commonly used dump bailers in the industry. One reason for this is its simplicity. However, gravity dump bailers present many drawbacks. Chief among them is the possibility of “stringing,” which occurs when the cement slurry does not completely discharge at the desired depth and the cement slurry is strung out through the well. Additionally, most gravity dump bailers include a seal, such as a ceramic disk, that is broken to allow the cement slurry to flow. The seal can be broken by a pin or, more frequently, shattered by an explosive charge. Positive displacement dump bailers address many of the deficiencies of the gravity dump bailers by elimination of the explosive charge and by providing force to expel the cement slurry out of the bailer.
There are several types of positive displacement dump bailers. Most positive displacement dump bailers rely on a sweep piston use to force the cement slurry or material out of the bailer. These systems may use a weight, either alone or with some actuating system, to force the piston down the bailer or the systems may use the pressure differential between atmospheric (well bore) pressure and the internal tool pressure to push the piston down the length of the bailer. While the positive displacement bailers overcome many of the deficiencies of the gravity dump bailers, they have several drawbacks. One of the main drawbacks is the use of bailer tubes, which hold the cement slurry. Because the sweep piston is forced through the bailer tubes, the bailer tubes must have a consistent inner diameter with a smooth wall bore to prevent the sweep piston from becoming lodged in the bailer tube and to reduce the friction between the pipe wall and the cement slurry. Additionally, because multiple bailer tubes are typically used, care must be taken not to damage the threaded connections. If the threaded connections are over tightened, the inner diameter of the bailer tube could neck down, causing the sweep piston to hang up.
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